1. Field of the Invention
This invention relates to reductive chlorination processes, and, in particular, to a method of producing an improved solid carbonaceous reductant whose use results in substantially decreased levels of environmentally unacceptable chlorinated hydrocarbons (C.sub.x Cl.sub.y), such a polychlorinated biphenyl (PCB), and hexachlorobenzene (HCB) in metal chlorides such as aluminum chloride produced therefrom.
2. Description of the Prior Art
The rate of carbon consumption is an important factor in a number of metallurgical processes. For example, in the production of anhydrous aluminum chloride, from aluminous raw material, as a precursor for producing aluminum, carbon or carbon monoxide is used as a reductant according to the following equations: EQU Al.sub.2 O.sub.3 +3/2C+3Cl.sub.2 =2AlCl.sub.3 +3/2CO.sub.2 ( 1) EQU Al.sub.2 O.sub.3 +3CO+3Cl.sub.2 =2AlCl.sub.3 +3CO.sub.2 ( 2)
The rate of reaction (2) is significantly higher than reaction (1). However, the use of carbon monoxide as a reductant in reaction (2) requires the generation of carbon monoxide from carbon. Thus, for either chlorination reaction, the activation of carbon is desirable to both increase the rate and/or lower the reaction temperature. Carbon monoxide as a reductant results in a rapid chlorination rate and produces an AlCl.sub.3 product with essentially no environmental contaminants such as chlorinated hydrocarbons including polychlorinated biphenyls (PCB's) or hexachlorobenzene (HCB), decachlorobiphenyl (DCBP), pentachlorobenzonitrile (PCBN), pentachloropyridine (PCP) and octachlorostyrene (OCS). Its cost, however is site dependent. Moreover, use of CO requires a mole ratio C/Al.sub.2 O.sub.3 of 3 which translates to about 0.67 lb C/lb Al and requires a significant gas volume to be handled resulting in higher capital cost. Solid reductants can result in stable cost regardless of site location and can result in lower off-gas volume depending on chlorination temperature.
Petroleum coke is a known source for solid carbon reductants for the chlorination of aluminous materials, such as partially calcined alumina (PCA), metal grade alumina (MGA) and partially calcined aluminum chloride hexahydrate (ACH). Green petroleum coke, i.e., uncalcined coke, is known to have a moderate level of activity. However, a serious disadvantage is that it contains significant quantities of hydrogen and hydrocarbons which are chlorinated during the chlorination process. Not only does their presence result in increased consumption of expensive unrecoverable chlorine, but the hydrogen and hydrocarbons interfere with the chlorination reaction kinetics due to their vapor pressure over the surface of the solid reductant. Calcining coke drives off the hydrogen and hydrocarbons but to a great extent also deactivates the carbon. Fully calcined coke, i.e., coke calcined at about 1200.degree.-1400.degree. C., has very low activity insofar as chlorination reaction kinetics are concerned, but is essentially free of excess hydrogen and hydrocarbons. Accordingly, methods of activating partially and fully calcined coke have been sought after for some time.
Petroleum coke is normally fully calcined to 1200.degree.-1400.degree. C. (FCC) to remove moisture and volatiles. The high temperature for calcination of coke is essential for use in aluminum chloride production since low temperature calcined coke contains residual hydrocarbons and hydrogen that can be converted, during chlorination, to environmentally unacceptable chlorinated hydrocarbon products, such as PCB's. However, the high temperature calcination of coke, sometimes called dead burning, produces coke with a low surface area and low activity.
An alternative to both carbon monoxide and fully calcined coke (FCC) is the use of traditional partially calcined coke (PCC) as taught and described in U.S. Pat. No. 4,284,607. This known method of producing partially calcined coke involves subjecting green petroleum coke in a nitrogen or non-oxidizing atmosphere to a calcination temperature of from 650.degree. C. to 900.degree. C. and preferably 850.degree. C. for a time period of 10 to 120 minutes, preferably 30 minutes. Typically, the calcination, alone or in conjunction with the aluminous materials to be chlorinated, is performed in a fluidized bed with nitrogen as the fluidizing agent of fluidization gas.
Traditional partially calcined coke (PCC), such as that of U.S. Pat. No. 4,284,607, is a relatively active reductant which results in rapid chlorination of aluminous materials such as metal grade alumina (MGA) to produce anhydrous aluminum chloride. However, a serious disadvantage of partially calcined coke presently known and used as a reductant for chlorination reactions is that it contains relatively high levels of residual hydrocarbons. During chlorination utilizing such partially calcined coke as the reductant, the residual hydrocarbons will also be chlorinated to produce environmentally harmful hydrocarbons such as polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB), decachlorobiphenyl (DCBP), pentachlorobenzonitrile (PCBN), pentachloropyridine (PCP), and octachlorostyrene (OCS).
Accordingly, a highly desirable and heretofore unknown solid carbon reductant would be one with the relative activity of known or traditional partially calcined coke but without the disadvantages present due to residual hydrocarbons which produce unacceptable chlorination products.